Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle span and low self-discharge ratio are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EV) and hybrid electric vehicles (HEV) as substitutes for vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are a major cause of air pollution. Nickel-metal hydride (Ni-MH) secondary batteries are generally used as power sources of electric vehicles (EV) and hybrid electric vehicles (HEV). However, a great deal of study associated with use of lithium secondary batteries with high energy density, discharge voltage and power stability is currently underway and some are commercially available.
In particular, lithium secondary batteries used for electric vehicles should have high energy density, exert high power within a short period of time and be useful under severe conditions for 10 years or longer, thus requiring considerably superior stability and long-period lifespan, as compared to conventional small lithium secondary batteries.
Conventional lithium secondary batteries used for small-size batteries generally utilize lithium cobalt composite oxide having a layered structure for a cathode and a graphite-based material for an anode. However, lithium cobalt composite oxide is disadvantageous in that cobalt used as a main element is extremely expensive and lithium cobalt composite oxide is unsuitable for use in an electric vehicle in terms of stability. Accordingly, lithium manganese composite oxide having a spinel crystal structure composed of manganese, which is low-cost and exhibits superior stability, may be suitable as the cathode of lithium ion batteries for electric vehicles.
However, when lithium manganese composite oxide is stored at a high temperature, manganese is eluted into an electrolyte, deteriorating battery properties. Accordingly, there is a need for measures to prevent this phenomenon. Further, lithium manganese composite oxide disadvantageously has a small capacity per battery weight, as compared to conventional lithium cobalt composite oxides or lithium nickel composite oxides, thus limiting an increase in capacity per battery weight. Batteries to improve this limitation should be designed in order to practically apply batteries as power sources of electric vehicles.
In order to solve these disadvantages, a great deal of research associated with fabrication of an electrode using a mix cathode active material is conducted. For example, Japanese Patent Application Publication Nos. 2002-110253 and 2004-134245 disclose a technology using a mixture of lithium manganese composite oxide and lithium nickel cobalt manganese composite oxide in order to increase revitalization power or the like, but still having disadvantages of poor cycle lifespan of lithium manganese oxide and limitation of improvement in stability.
Further, Korean Patent No. 0458584 discloses a cathode active material composed of an active material compound having a nickel-based large spherical diameter with an average diameter of 7 to 25 μm and an active material compound having a nickel-based small spherical diameter with an average diameter of 2 to 6 μm (for example LIxMn2O4−zXz, in which X represents F, S or P and 0.90≦x≦1.1 and 0≦z≦0.5) to increase the volume density of an electrode plate and thereby improve battery capacity.
In addition, in order to improve battery capacity, lifespan and high-rate discharge properties, Korean Patent No. 0570417 discloses use of lithium tetraoxide dimanganese having a spinel crystal structure as a cathode active material, Japanese Patent Application Publication No. 2002-0080448 discloses use of a cathode active material containing lithium manganese composite oxide and Japanese Patent Application Publication No. 2004-134245 discloses fabrication of secondary batteries using a cathode active material containing lithium manganese composite oxide having a spinel crystal structure and lithium transition metal composite oxide.
However, in spite of this related art, secondary batteries having suitable lifespan and stability have yet to be developed.